class Person(object):
    def __init__(self, name, gender):
        self.name = name
        self.gender = gender

class Student(Person):
    def __init__(self, name, gender, score):
        super(Student, self).__init__(name, gender)
        self.score = score
    def __tr__(self):        //注意此处的str变成了tr
        return '(Student: %s, %s, %s)' % (self.name, self.gender, self.score)
    __repr__ = __tr__     //若这句存在则能正常输出，若这句不存在则输出错误，为什么呢？
s = Student('Bob', 'male', 88)
print s